Various types of networks, for example electrical networks and electrical connection lines used for communication purposes, for example telephone, telex, facsimile and the like, frequently have to be tested not only when running new lines, but also upon connection of additional substations or subscribers to the network. Routine testing should be necessary of such additional or new connections. Continuity testers are well known, in which an electrical connection between two measuring points can be tested. The continuity tester can also determine if one line extends from one test point to another test point and, further, if that line has an electrical connection to a third line or network. To test such lines, it has been known to use audio frequency generators and receivers. The audio frequency generator supplies electrical energy at an audio frequency, for short, an "audio frequency", into the network to be tested, for short the "check network". Use of an ordinary audio frequency receiver, connected to the check network, then permits obtaining information which lines or network portions have the particular audio frequency applied thereto. Galvanic connection between the audio frequency generator and the receiver is not necessary; inductive transmission, capacitative coupling, or intermediate amplifiers which, for example, may include opto-electronic couplers, can be used.
Test apparatus which include a continuity tester, an audio frequency generator and/or an audio frequency receiver are voluminous, heavy and expensive. Such apparatus, frequently, is not suitable for use at test locations which are not readily accessible. Known test apparatus, particularly if the lines are subject to possible stray connected energy, may be damaged, particularly when earphones are used, or sensitive electronic equipment is employed. The networks may, for example, be subject to voltage peaks and/or d-c energy components, inadvertently or erroneously applied to the networks, which may cause damage not only to the test apparatus but also to other apparatus which may be connected to the networks or may have been connected to the networks already before additional connections were made thereto.